Radio transmission of information signals takes place by modulating the information signals on a radio frequency (RF) wave known as the RF carrier. Common modulation methods include Amplitude Modulation (AM), Frequency Modulation (FM), and Phase Modulation (PM). In AM, the amplitude or strength of the RF carrier is varied in relation to the information signal, while the freqency of the carrier itself remains constant. In FM and PM, the frequency and phase, respectively, of the RF carrier are varied while the amplitude remains constant.
For constant amplitude modulation techniques, such as FM and PM, efficient radio frequency transmitters of simple construction may be designed to achieve maximum efficiency at a single, constant amplitude. In contrast, because the RF carrier amplitude varies considerably in amplitude modulation, efficient AM transmitters are more difficult to construct and are usually less efficient than constant amplitude transmitters. For complex modulation schemes, such as Single Side Band (SSB) and Quadrature Phase Shift Keying (QPSK), where both the amplitude and the phase of the carrier are modulated, transmitters capable of transmitting complex modulated signals are the most difficult to construct. Consequently, when a single transmitter is designed to transmit signals that in a first mode are constant amplitude FM or PM signals, and a second mode are amplitude modulated or complex modulated signals, the provision of an amplitude modulation capability for the second mode adversely affects the transmitter efficiency in the first mode.
The conventional solution for efficiently generating a high-power, amplitude modulated signal is to amplify a low frequency, modulating signal initially to a high power level up to half of the transmitter mean output power. The high power modulating signal is added in series with a DC power supply to the transmitter power amplifier in order to produce a modulated power supply voltage and hence a modulated power output. This procedure is called high-level modulation.
Of the various classes of power amplifiers, Class A, AB, B, or C, RF power amplifiers using high-level modulation operate at maximum efficiency in the non-linear, Class C mode of operation. However, the penalty for obtaining high amplifier efficiency is that transmitters that use high level modulation are large, heavy, and costly.
In applications where size, weight or cost considerations are more important than efficiency, such as portable, radiotelephone transmitters, low-level modulation techniques must be used. Low-level modulation techniques initially amplify the carrier to a low power level at which amplifier efficiency is immaterial. After the carrier is modulated, however, it must be amplified to a high power level using a linear power amplifier. Unfortunately, only lower efficiency Class A, AB, or B power amplifiers are capable of reasonably linear operation. Thus, high efficiency, Class C amplifiers can not be used in mobile radiotelephone transmitters that employ low-level amplitude modulation.
Class A amplifiers conduct current for an entire input cycle and therefore consume power continuously even when output power is not required. As is well known, the maximum theoretical efficiency of a Class A amplifier at full output power is 50%. However, because AM signals operate at a mean power equal to 1/4 the peak output power, the maximum theoretical efficiency at mean output power is only 12.5%. The failure to reduce power consumption for a corresponding reduction in power required results in this low efficiency. Fortunately, this disadvantage is partially overcome by a Class B amplifier.
Conventional, Class B amplifiers include two identical amplifiers arranged in a push-pull fashion and biased so that current is not consumed when output power is not required. When a sinusoidal, RF input signal enters a positive cycle, one of the amplifiers conducts current. Similarly, the other amplifier conducts current on the subsequent negative half cycle. Thus, each amplifier amplifies one-half of the sine-shaped current signal such that the mean output current equals 1/Pi times the peak output current. Accordingly, the total mean current for both amplifiers is 2/Pi times the peak output current. As the RF input signal level increases, the current delivered to the load increases proportionally until the output voltage developed across the load impedance is equal to the power supply voltage. At this point, any further increase in the RF input signal drives the amplifier into saturation. For linear modulation, driving the power amplifier into saturation distorts the modulated signal. For this reason, the amplifier must not be driven into saturation.
To achieve maximum efficiency in Class B, linear power amplifiers, the amplifier must be operated at the point just before saturation where voltage across the load equals the power supply voltage. The ratio of the output power to the power consumed at peak output current results in the theoretical maximum efficiency of Pi/4 or 78.5%. At less than peak amplitude, however, efficiency is determined based on two factors: (1) power consumption decreases in proportion to the output voltage and (2) power output decreases in proportion to the square of the output voltage. Consequently, the efficiency decreases linearly with the output voltage or amplitude. At a mean output voltage equal to one-half of the peak output voltage, the mean efficiency is equal to one-half the peak efficiency, i.e., 0.5 Pi/4 or 39%.
As is evident from the description above, the efficiency of Class B amplifiers is considerably better than that of Class A amplifiers at mean amplitude output level. The efficiency improves because Class B amplifiers reduce current consumption in proportion to the power demand. Nonetheless, the mean amplitude efficiency of 39% for AM transmission is low compared to the peak amplitude/full output efficiency of 78.5% for constant amplitude modulation.
The problem addressed by the present invention is the construction of a power amplifier that transmits both amplitude modulated and angle modulated signals at the same mean power output level without a significant reduction in efficiency. For example, if a Class B amplifier is used to transmit a one-watt carrier using AM transmission, the amplifier must amplify the modulation peaks so that four watts of peak power is output. If the same amplifier is then used to transmit a one-watt carrier using FM transmission, its efficiency would only be 39% instead of the 78.5% theoretically possible for FM transmissions. On the other hand, if the full power of 4 watts is used in the FM mode, the efficiency would be 78.5%, but the total power consumption would rise to twice that consumed in the 1-watt situation.
In battery-operated equipment, such as handheld radiotelephones, operating the power amplifier either at reduced power and half efficiency or at full power and full efficiency, has an equally negative impact on battery life. Therefore, a goal of the present invention is to construct an efficient power amplifier circuit that efficiently transmits AM signals at one power level and efficiently transmits angle modulated signals at other power levels.